PROJECT SUMMARY/ABSTRACT Increases in children's standardized BMI (BMIz) have been demonstrated during summer across the US and internationally. The largest study to demonstrate this was our 5-year longitudinal study (kindergarten ? 5th grade) which demonstrated that beginning in early elementary school, children begin a consistent pattern of improvements in BMIz during the school year and significant increases in BMIz during summer, with summertime increases in BMIz contributing to increased risk of becoming overweight or obese by 5th grade. Shortened sleep duration and circadian rhythm disturbances have been clearly implicated in obesity onset. Specifically, shifts in the daily behavioral rhythms of sleep and meal patterns are associated with increased adiposity, mediated through subsequent alterations in the molecular circadian clock. The transition from the school year to summer vacation represents a time during which children may experience changes in their behavioral rhythms (i.e., sleep and behavioral rhythms). Interpersonal Social Rhythm Therapy (IPSRT) is an evidence based treatment for bipolar disorder that has demonstrated efficacy to prevent the reoccurrences of mood disturbances by preventing the misalignment of endogenous circadian rhythms through the promotion of stable behavioral rhythms (e.g., sleep and meal patterns). The goals of the proposed research are to 1) adapt IPSRT to promote stable behavioral rhythms in children during summer vacation for the prevention of summer increases in BMIz (i.e., the i?rhythm project) and 2) conduct a pilot study to examine the feasibility, acceptability, and preliminary efficacy of the i?rhythm project to prevent increases in children's BMIz during summer. mHealth technologies will be utilized in order to reach parents using a familiar and convenient resource during out of school times. The K99 phase will provide additional training in order to develop expertise in sleep and circadian rhythms research and the development of behavioral health interventions using mHealth technologies, as well as additional professional and research development needed to address gaps in training. The additional training will facilitate transition to a career focused on the development of behavioral interventions to address the role of sleep and circadian rhythms in the onset of childhood obesity. The proposed research addresses several gaps in scientific knowledge and practice: 1) there are a paucity of interventions addressing summer weight gain in children 2) there is a significant gap in knowledge regarding the role of sleep and circadian rhythms in the prevention of child obesity and 3) addressing summer weight gain provides an important opportunity to address the prevention of child obesity. This innovative application represents a novel application of an intervention with established efficacy to promote stable behavioral rhythms for treatment of bipolar disorder and promises to do the same for the prevention of child obesity. This research is the first to address the role of sleep and circadian rhythms in the prevention of child obesity and to examine the preliminary evidence of such an intervention on the long-term prevention of obesity.